It is known to place cementitious mixtures in underground spaces for disposal and for providing resistance to potential subsidence. For example, it is known to use cementitious compositions, including flue gas desulfurization (FGD) sludge, fly ash, lime and water in this manner to provide resistance to potential subsidence. Typically, such compositions are transported into underground tunnels or mine "rooms" through vertical boreholes.
Borehole placement is dependent on the flow characteristics of the cementitious composition. For example, a composition having a relatively low viscosity (i.e., better flow characteristics) will require fewer boreholes to achieve the same void space coverage compared to a composition having a relatively higher viscosity with more boreholes. Maximizing coverage optimizes prevention of subsidence. However, the requirement for a relatively large number of vertical boreholes is a significant cost factor, minimizing the use of this otherwise environmentally and technically desirable concept.
Higher water content also renders cementitious mixtures more readily flowable to facilitate subsurface placement. Addition of water typically reduces viscosity, permits easier pumping and results in greater dispersion to subsurface voids. However, high water content compositions tend to separate after placement, leaving voids in the subsequently hardened composition. This necessitates multiple cycles of placement, separation and hardening to obtain the final fill-up condition. Additionally, higher levels of water are detrimental to the cementitious properties of the FGD waste-containing compositions. Thus, simple addition of water is not an appropriate remedy for this problem.
Unoxidized FGD wastes, which are primarily calcium sulfite as opposed to calcium sulfate, are known to tend toward plasticity or fluidity when vibrated or agitated, and then not regain a solid form. This tendency, historically, has been a negative factor in handling unoxidized FGD waste, as this characteristic results in several plant operational difficulties.
For example, an integral process in FGD waste disposal is "thickening," which is typically a primary dewatering stage from an FGD scrubber. Thickening involves depositing scrubber discharge, usually at about 5 to 10% solids, in a circular vessel with a shallow, conical bottom to allow some settling. Clear supernatent flows over the top lip of the vessel, thereby increasing the solids content of the thickener underflow. Pumping scrubber discharge from an FGD scrubber to a thickener with a pump, which imparts a high-shear force on the fluid, inhibits the calcium sulfite FGD wastes of the scrubber discharge from settling.
A similar problem occurs in the secondary dewatering stage from an FGD scrubber, typically a vacuum filter. In this stage, depending on the quality of material, the solids content exiting the secondary dewatering stage might be from 45 to 80% solids (with a composition having 25 to 50% solids exiting the primary dewatering stage). It has been found that pumped wastes are more difficult to filter.
If a composition is not sufficiently dewatered by the primary and secondary dewatering stages, several problems result. First, material is difficult to handle in this fluidized, flowable form. Further, the slightest mechanical agitation (e.g., by front end loaders or during trucking) reduces the material to its flowable state, thereby making it difficult to handle. Even after placing such material in a landfill, a problem exists. In particular, rubber-tired vehicles encounter difficulty in crossing such a surface, because these vehicles lose traction by tires applying shear stress and agitating the material to return it, on a localized basis, to the flowable state.
Although the fluid flow properties of oxidized FGD wastes (primarily calcium sulfate) are less problematic in general, the above-discussed flow characteristics for unoxidized FGD wastes (primarily calcium sulfite) are notorious. Attempts have been made to avoid the problems mentioned above caused by the shear-induced reduction in viscosity during the processing of FGD sludge. For example, it has been found that positive displacement pumps, which gently transport the material from stage to stage, minimize the shear-induced thixotropic effect.